A small “hack” has been released into the wild which will allow you to enable tethering on your iPhone running the 3.0 OS without jailbreaking the device. Full directions for enabling tethering are below:
What you need:
1. Mac OS X 10.5 or later
2. Download this carrier settings file from gizmodo.com
3. iPhone OS 3.0 (7A341)
4. iTunes 8.2
5. USB cable that came with your iPhone
1. Double click the iphonetetheringfile.dmg, open Finder, drag the ATT_US.ippc file to desktop.
2. Open iTunes and open the Terminal.
3. In the Terminal paste the following line and hit enter.
defaults write com.apple.iTunes carrier-testing -bool TRUE
4. Plug in your iPhone running the 3.0 OS, let it sync, once it says it is OK to disconnect hold down your Options key (Alt on a Windows keyboard) and while holding down the key click Restore.
5. When the dialog box pops up go to the Desktop and click the ATT_US.ippc file (it should not be grayed out, if it is do step 3 again), then click Ok.
6. Now your iPhone will update with the latest settings. This should take less than one minute. Once it’s done updating you should be all set.
Keep your iPhone plugged in and navigate to the tethering option on the iPhone which can be found under Settings, General, and Network. If you click Internet Tethering and it says you need to contact AT&T, click through it and click Internet Tethering again, it should go through to the next page where you can turn it on.
Click Network Preferences… and then select your new ethernet device and click “Apply”. That will activate your network connection and assign an IP.
In order to indicate tethering is on your iPhone will show a blue bar at the top which clearly states it’s on (see picture below).